1. Field of the Invention
The present invention generally relates to fixtures for holding workpieces during lapping and polishing operations and more specifically to an improved fixture which accommodates different sized workpieces.
2. Description of the Related Art
Ceramic substrates routinely go through a process of lapping and/or polishing, after they have been fired, to achieve desired flatness, roughness and thickness specifications. These processes involve the fixturing of the substrate in a dressing wheel or polishing head which is then turned on a spinning table upon which abrasive slurries are applied.
FIG. 1 illustrates a conventional double-sided lapping machine. Motor 1 turns gearbox 2 to drive an upper dressing/working wheel 3. Similarly, motor 8 turns gear box 9 to drive a lower dressing/working wheel 7, via a belt 10. A workpiece to be lapped (not illustrated) is placed in a fixture (not illustrated), between two working wheels 3 and 7. Motor 4 and belt 6 turn gear box 5 to drive a workpiece drive 11.
A large wheel 20 (shown in FIG. 2), typically made of spring-steel that contains round openings 21, is mounted on the workpiece drive 11. The workpiece drive 11 turns the large wheel 20. Plastic fixtures 22 (e.g., disks) are inserted into these round openings 21. It is these fixtures 22 that are the subject of this invention.
FIG. 3 shows a conventional fixture 30 that holds two ceramic substrates 40 (FIG. 4) for lapping or polishing in separate openings 31. The fixture 30 holds the substrate 40 in place and must be fabricated to fit the individual substrates 40. The fixture 30 can be made of any machinable material, such as polyvinyl chloride (PVC), plastic or metal.
The fixture 30 must be thinner than the workpiece or substrate 40 and thin enough to allow for sufficient removal of material from the workpiece 40. Openings 31 are typically fabricated to a tolerance of -0.000"/+0.005" of the overall length 41 and width 42 of the workpiece 40 to be lapped or polished. This tolerance is required to minimize the amount of movement within the opening 31. Too much movement results in chipping of the workpiece edges during lapping and polishing.
If the opening 31 in the fixture 30 is too small, the substrate 40 will not fit inside the fixture 30. If the opening 31 in the fixture 30 is too big, the substrate 40 will move around too much and will result in "chipping" on the substrate.
This invention solves the problem that occurs when the length 41 and width 42 of the substrate 40 varies more than the tolerance of the fixtures 30. This usually occurs when the workpiece 40 is to be lapped and/or polished after sintering. In other words, the length 41 and/or the width 42 of the workpiece 40 commonly varies by more than 0.005".
Conventional solutions to this problem include a sizing operation between sintering and lapping or polishing. In this manner, all parts (e.g., substrates 40) would be accurately cut to fit the opening 31. However, this is an expensive solution. Another conventional solution is making multiple sized openings 31 and "best fitting" the parts 40 to the openings 31. However, this process is time consuming and difficult.